Roentgen discovered X-radiation by the inadvertent exposure of a silver halide photographic element. In 1913, Eastman Kodak Company introduced its first product specifically intended to be exposed by X-radiation (X-rays). Silver halide radiographic films account for the overwhelming majority of medical diagnostic images. It was recognized almost immediately that the high energy ionizing X-rays are potentially harmful, and ways were sought to avoid high levels of patient exposure. Radiographic films provide viewable silver images upon imagewise exposure followed by rapid access processing.
One approach, still in wide-spread use is to coat the silver halide emulsions useful in radiographic films on both sides of the film support. Thus, the number of X-rays that can be absorbed and used for imaging are doubled, providing higher sensitivity. Dual-coated radiographic films are sold by Eastman Kodak Company under the trademark DUPLITIZED.TM. films. Films that rely entirely on X-radiation absorption for image capture are referred to in the art as "direct" radiographic elements, while those that rely on intensifying screen light emission are referred to as "indirect" radiographic elements. Because the silver halide emulsions are used to capture the X-rays directly in "direct" films, the silver coating coverages of direct radiographic elements are generally higher than for indirect radiographic elements.
Among the "direct" radiographic films, are films most commonly used for dental intra-oral diagnostic imaging and hereafter referred to as dental films. Intra-oral dental imaging presents obvious barriers to the use of intensifying screens. Thus, dental films utilize the coated silver halide to absorb X-rays.
There are other applications for direct radiographic films, such as in various industrial applications where X-rays are captured in imaging, but intensifying screens cannot be used for some reason.
U.S. Pat. No. 5,370,977 (Zietlow) describes dental films having improved characteristics and containing certain tabular grain silver halide emulsions. No spectral sensitization is used in such dental films, but in order to avoid fogging the films with inadvertent light exposure, the emulsions contain what is identified as a "desensitizer" that reduces emulsion sensitivity to light. Conventional processing solutions and conditions are described for these dental films.
Other desensitizing compounds for radiographic films are described in U.S. Pat. No. 3,630,744 (Thiers et al) for reducing film sensitivity to roomlight and UV radiation. Conventional processing of these films is also described.
Dual-coated indirect radiographic elements described in U.S. Pat. No. 4,803,150 (Dickerson et al) contain certain microcrystalline particulate dyes that reduce "crossover". These elements are designed for use with intensifying screens. Crossover occurs when some light emitted by the screen passes through the film support and exposes silver halide grains on the opposite side, resulting in reduced image sharpness. The noted particulate dyes absorb unwanted crossover exposure, but can be decolorized during conventional processing. Thus, a pH 10 developing solution is described for its conventional use as well as to decolorize the dyes within 90 seconds. Conventional fixing and washing follow.
It is the prevailing practice to process direct radiographic films for 3 or more minutes because of the higher silver coating coverages. Such processes typically include black-and-white development, fixing, washing and drying. Films processed in this manner are then ready for image viewing.
Photographic developing solutions containing a silver halide developing agent are well known in the photographic art for reducing silver halide grains containing a latent image to yield a developed photographic image. Many useful developing agents are known in the art, with hydroquinone and similar dihydroxybenzene compounds and ascorbic acid (and derivatives) being some of the most common. Such solutions generally contain other components such as sulfites, buffers, antifoggants, halides and hardeners. A workable pH for such solution is usually in the range of from about 10 to about 11, depending upon the developing agent and other solution components.
Fixing solutions for radiographic films are also well known and include one or more fixing agents, of which thiosulfates are most common. Such solutions generally include sulfites as antioxidants, and hardeners, and have a functional pH range of from about 4 to about 5.5.
Direct radiographic films, including dental films, thus have some sensitivity to roomlight and UV as well as X-rays, and therefore care must be taken to avoid inadvertent room-light exposure before and during processing. There has been a desire for radiographic films that are less sensitive to roomlight, and that can be handled and processed without the need for a darkroom or other special conditions. Such films would have a number of useful applications, such as dental and industrial imaging. However, conventional processing solutions and methods cannot be used to provide suitable radiographic images in such films.
"Monobath" solutions are also known in the art of photographic processing. These solutions typically require long processing times and contain components common to both developing and fixing compositions, that is a high pH and sulfite.
U.S. Pat. No. 5,866,309, filed Oct. 22, 1997, describes the use of separate developing and fixing compositions for roomlight processing roomlight-handleable films, including radiographic dental films in sequential processing steps. While those compositions represent an advance in the art, they must be separately balanced in pH in relation to each other so that the light protecting dyes and desensitizers are not deactivated prematurely. Specifically, the developing composition has a pH and sulfite concentration lower than the fixing composition. Thus, the developing composition activity is limited, and is more complicated than a "monobath" process.
U.S. Ser. No. 09/046,449, noted above, describes "opaque" monobath compositions containing particulate opacifying agents, such as carbon black. Such dilute carbon black compositions adequately block light during processing and remain sufficiently dispersed to process black-and-white films within 60 seconds.
While this processing system is effective for providing black-and-white images within 60 seconds, it has a disadvantage in that the particulate opacifying agent (for example, carbon black) is difficult to completely remove from the surfaces of the processed films. In addition, because of the opaque nature of the processing composition, it is difficult to observe the progress of development by looking at the solution.
A technology is needed wherein black-and-white elements can be handled and processed completely in roomlight, with the attendant advantages of known processing systems, but which avoids the problems noted above from the use of carbon black or other opacifying agents.